Coating compositions and methods for coating substrates with cured coating compositions have been known for years. A coating composition is typically applied as a film which may include water and/or organic solvent. The coating is typically applied on a substrate by a process such as lamination, solution or dispersion coating, powder coating, electrocoating, spraying, roll coating, or reverse roll coating. Coating compositions can also be applied as a film by extrusion in melt form through an extrusion coating die onto a substrate. Once coated to the substrate, the composition must dry and cure without defects within a few seconds as modem high speed coating lines heat the coated substrates rapidly to temperatures of up to 490.degree. F. (254.degree. C). Many different types of substrates have been coated using such techniques, including wood, plastics, and metal in the form of sheets, strips or coils. Metal coated substrates are especially useful in the packaging, processing, and holding of foods and beverages.
It is desirable to avoid the use of excess organic solvent in a coating method so that the environmental hazards of allowing organic solvent to evaporate into the atmosphere are minimized. However, a relatively large quantity of organic solvent is typically required in order to process a coating composition and provide a coalesced film after drying.
A number of attempts have been made in the prior art to prepare aqueous, solvent based, or solvent free coating compositions suitable for use in a process for forming cured coated substrates. For example, a water-borne coating composition prepared by combining an epoxy resin, a carboxylic acid polymer, and an excess of tertiary amine to form a reaction product has been reported. This excess of amine provides a resin coating composition wherein polymeric quaternary ammonium salt linkages almost completely predominate over ester linkages. This process is carried out in a mixed organic and aqueous medium. In another variation, the reaction of an .alpha., .beta.-ethylenically unsaturated carboxylic acid with some of the oxirane groups prior to the formation of quaternary ammonium groups using a preformed carboxylic acid containing addition polymer has been described. Another solution based coating composition, which includes an aqueous epoxy ester emulsion prepared by forming a carboxyl bearing polymer in the presence of a deflinctionalized epoxy resin in an organic solvent has been reported.
Solvent-free low-monomer or monomer-free hot melt compositions polymerizable by means of free radicals and suitable for use in coating various substrates have also been described. The compositions do not include quaternary ammonium compounds.
Conventional coating compositions containing quaternary ammonium salts typically suffer from the formation of extremely high molecular weight resins with high viscosity. Such compositions often possess such a high viscosity that application to a substrate can be hindered, resulting in problems of uneven flow and poor adhesion qualities. One other problem that commonly arises during attempted preparation of a water based coating composition is formation of an extremely high molecular weight product that will not dissolve in water. There is a continuing need for improved low viscosity coating compositions that can be applied to a substrate with excellent coating characteristics.